Copper-based alloys and composites are candidate materials for high heat flux structural applications because of their high thermal conductivity and high-temperature strength. Such applications include hot gas walls for combustion chambers and surfaces of nozzle ramps for rocket engines and for the next generation launch vehicles. Other applications include protective coatings for heat exchangers in commercial power generation systems and firearm barrels.
Such applications for these type of services have employed many copper-based materials. Examples of copper-based materials include Cu—Ag—Zr, Cu—Be, Cu—Co—B, Cu—Cr, Cu—Cr—Al, Cu—Cr—Nb, Cu—Cr—Zr—Mg, Cu—Nb, Cu—Ni, Cu—Ta, Cu—Zr, and Cu—Zr—Ti alloys, and oxygen-free high conductivity (OFHC) copper. Since the 1970s, an alloy commonly referred to as NARLOY-Z (Cu-3 wt. % Ag-0.5 wt. % Zr) has been the predominantly chosen material for use in the high-temperature, oxidative environments present in certain rocket components, such as the space shuttle main engine (SSME). In recent years, however, GRCOP-84 (Cu-8 atom % Cr-4 atom % Nb), developed at the NASA Glenn Research Center, has become a candidate material for various high temperature applications due to its many superior properties, including thermal expansion, yield strength, and strength retention following simulated brazing. (See, e.g., Ellis, David L.; and Michal, Gary M.: Mechanical and Thermal Properties of Two Cu—Cr—Nb Alloys and NARloy-Z. NASA CR-198529, 1996).
A major limitation to the use of any of these copper-based materials, however, is their rapid oxidation at elevated temperatures. In addition, copper-alloy rocket engine combustion chamber linings have been found to deteriorate when exposed to cyclic reducing/oxidizing (redox) environments, which are a consequence of the combustion process. This deterioration, known as blanching, can be characterized by increased roughness and burn-through sites in the wall of the combustion chamber lining and can seriously reduce the operational lifetime of the combustion chamber.
An illustrative example of a copper alloy rocket engine combustion chamber that undergoes blanching is in the SSME propulsion system. A high pressure, high temperature rocket engine, the SSME burns a mixture of liquid oxygen and liquid hydrogen. During combustion, localized regions along the combustion chamber's wall lining become, alternatively, rich in oxygen (forming an oxidizing environment) and rich in hydrogen (forming a reducing environment). When a region of the combustion chamber's lining is exposed to an oxidizing environment, copper oxides form. Subsequently, when exposed to a reducing environment, these copper oxides are reduced. The result of cycling a region of the chamber wall between an oxidizing and reducing environment is to cause the wall lining to become scarred and rough. This, in turn, can result in localized hot spots that reduce the operational (i.e., useful) lifetime of the combustion chamber.
Oxidation resistance requirements for useful protective coatings vary for specific applications. The maximum surface temperature of combustion chambers, nozzles, and actively cooled structures is expected to be below 600° C. Each “cycle” of exposure consists of approximately 8 minutes at maximum temperature. The coating is expected to last a minimum of 60 such cycles. For aerospace vehicle applications, the coating is expected to reach 650° C. in oxidizing environments. Each cycle therein consists of approximately 12 minutes at maximum temperature. The coating is expected to last a minimum of 20 such cycles. For gun barrel application, the maximum temperature is estimated to be 650° C. Each cycle therein consists of approximately 2 minutes at maximum temperature. The coating is expected to last a minimum of 10,000 such cycles.
A means of combating the oxidation and any subsequent blanching of copper-based alloys is to coat the surface thereof with a protective coating. Numerous coating materials and protocols for their application to copper-based alloys have been reported. For example, Beers, et al. (U.S. Pat. No. 6,277,499) describe a two-step process for protecting copper and copper-based composites from high temperature oxidation by the application thereto of a cobalt-based alloy diffusion barrier and a copper-aluminum alloy protective outer layer. The procedure disclosed by Beers, et al. involves a first step wherein a diffusion barrier comprising, (1) cobalt, chromium, nickel, carbon, tungsten, and manganese, or (2) cobalt, chromium, carbon, iron, tungsten, and niobium or tantalum, is applied to the surface of the copper-containing substrate by a method such as cathodic arc deposition. In a second step, a protective outer layer comprising Cu—Al (8 wt. % Al) is applied over the diffusion barrier by a method such as cathodic arc deposition.
Likewise, Holmes, et al. (U.S. Pat. No. 6,314,720) describe a rocket combustion chamber coating comprising a protective coating and a transitional layer between the copper-containing combustion chamber surface (lining) and the protective coating. The protective coating may comprise either a metallic or a ceramic material. Metallic compositions described as suitable for the protective coating consist of R1CrAlY, wherein R1 is nickel, cobalt, iron, or a mixture thereof. Suitable ceramic protective coating materials include zirconium oxide stabilized with yttrium oxide, mullite, alumina, zircon, hafnium carbide, hafnium diboride, and hafnium nitride. The transitional layer, as the name suggests, is a layer that comprises both the chamber lining material and the protective coating material, wherein the transitional layer comprises a composition gradient therebetween. In another embodiment, Holmes, et al. describe a process wherein a second protective coating is applied over the first protective coating, with a second transitional layer comprising a composition gradient of the two protective coatings therebetween. In both embodiments the layers are applied using a vacuum plasma spray process.
In addition, Raj (U.S. Pat. No. 6,838,191) discloses blanch-resistant NiAl coatings for copper alloys such as GRCOP-84 (Cu-8 atom % Cr-4 atom % Nb). The coatings described therein comprise a bond coat deposited on the copper alloy and a NiAl topcoat deposited thereover. Suitable bond coats described include Ni, Cu, and Cu—Cr alloys. Bond coats are applied via direct spraying of powder on the copper-alloy substrate or using low pressure or vacuum plasma spray techniques. The NiAl topcoat is deposited using low pressure or vacuum plasma spray.